The present disclosure is related to marking and printing systems, and more specifically to variably data lithography system employing an edge-writing thermal print head.
Offset lithography is a common method of printing today. (For the purposes hereof, the terms “printing” and “marking” are interchangeable.) In a typical lithographic process a printing plate, which may be a flat plate, the surface of a cylinder, or belt, etc., is formed to have “image regions” formed of hydrophobic and oleophilic material, and “non-image regions” formed of a hydrophilic material. The image regions are regions corresponding to the areas on the final print (i.e., the target substrate) that are occupied by a printing or marking material such as ink, whereas the non-image regions are the regions corresponding to the areas on the final print that are not occupied by said marking material. The hydrophilic regions accept and are readily wetted by a water-based fluid, commonly referred to as a fountain solution (typically consisting of water and a small amount of alcohol as well as other additives and/or surfactants to reduce surface tension). The hydrophobic regions repel fountain solution and accept ink, whereas the fountain solution formed over the hydrophilic regions forms a fluid “release layer” for rejecting ink. Therefore the hydrophilic regions of the printing plate correspond to unprinted areas, or “non-image areas”, of the final print.
The ink may be transferred directly to a substrate, such as paper, or may be applied to an intermediate surface, such as an offset (or blanket) cylinder in an offset printing system. The offset cylinder is covered with a conformable coating or sleeve with a surface that can conform to the texture of the substrate, which may have surface peak-to-valley depth somewhat greater than the surface peak-to-valley depth of the imaging plate. Also, the surface roughness of the offset blanket cylinder helps to deliver a more uniform layer of printing material to the substrate free of defects such as mottle. Sufficient pressure is used to transfer the image from the offset cylinder to the substrate. Pinching the substrate between the offset cylinder and an impression cylinder provides this pressure.
Typical lithographic and offset printing techniques utilize plates which are permanently patterned, and are therefore useful only when printing a large number of copies of the same image (long print runs), such as magazines, newspapers, and the like. However, they do not permit creating and printing a new pattern from one page to the next without removing and replacing the print cylinder and/or the imaging plate (i.e., the technique cannot accommodate true high speed variable data printing wherein the image changes from impression to impression, for example, as in the case of digital printing systems). Furthermore, the cost of the permanently patterned imaging plates or cylinders is amortized over the number of copies. The cost per printed copy is therefore higher for shorter print runs of the same image than for longer print runs of the same image, as opposed to prints from digital printing systems.
Accordingly, a lithographic technique, referred to as variable data lithography, has been developed which uses a non-patterned reimageable surface coated with dampening fluid. Regions of the dampening fluid are removed by exposure to a focused radiation source (e.g., a laser light source). A temporary pattern in the dampening fluid is thereby formed over the non-patterned reimageable surface. Ink applied thereover is retained over the surface in areas formed by the removal of the dampening fluid. The dampening fluid may then be removed, a new, uniform layer of dampening fluid applied to the reimageable surface, and the process repeated.
According to known systems, the patterning of dampening fluid on the reimageable surface in variable data lithography essentially involves using a laser to selectively boil off or ablate the dampening fluid in selected locations. This process can be energy intensive due to the large latent heat of vaporization of water. At the same time, high-speed printing necessitates the use of high-speed modulation of the laser source, which can be prohibitively expensive for high power lasers. Furthermore, the vaporized dampening fluid produces a “cloud” which may absorb laser energy and otherwise interfere with the laser patterning process. Still further, laser-based optical systems are relatively large, leading to relatively large marking systems. And laser writing systems require scanning and focusing optics which are susceptible to alignment inaccuracies affecting writing to the dampening fluid and ultimately affecting print quality.